Regenerative braking is the conversion of kinetic energy created by a system powered by a motor into electrical energy, and the storing of that electrical energy to power the motor.
Zapi Corporation of Italy manufactures direct current motor controllers using a regenerative braking circuit, as illustrated in FIG. 1. The essential element of the Zapi regenerative braking circuit is a single-pole, double-throw regenerative braking contactor 10. The regenerative braking contactor 10 is energized in drive mode. When energized, the regenerative braking contactor 10 allows current to flow from the positive terminal of battery 20 through armature winding 30, field winding 40, and motor drive FETs 50.
The regenerative braking contactor 10 is de-energized in braking mode. When the regenerative braking contactor is de-energized its contacts connect a top connector A1 of the armature winding 30 to the negative terminal of the battery 20. This connection enables regenerative braking.
While the Zapi circuit is somewhat effective, it may be utilized only under limited conditions. In fact, three conditions must be met in order for regenerative braking to occur: first, the motor field winding 40 must contain sufficient flux to induce an opposing current in the armature 30; second, the motor field winding 40 must be electrically connected such that the magnetic interaction of the armature 30 with the induced field generates a regenerative current (i.e., the direction contactors 42 and 44 must be reversed so the motor current flows in a direction that opposes the existing direction of motor rotation); and finally, a complete path for current flow must exist from the motor field winding 40 to the battery 20.
These conditions for operation make reversing the direction contactors 42 and 44 very difficult. For example, to switch from the forward driving direction to the reverse driving direction, the motor current must be off. Otherwise, if the direction contactors 42 and 44 are reversed while a substantial current is flowing through the motor, there is a danger the contactors will be welded in place. Moreover, even if the direction contactors 42 and 44 are successfully switched while current is flowing through the motor, the contactor switch times are generally longer than the time constant (the time required for flux dissipation) of high powered direct current motors. Thus, there will be little or no flux left in the field winding 40 after the contactors are reversed.
For regenerative braking to be safely induced using the available Zapi circuit, a secondary mechanism must be included for exciting the regenerative braking current in the armature 30. This is accomplished by using a weak magnetic field induced in the iron of the electric motor case (not shown). If a flux path is in the proper direction in relation to the magnetic field, a high rotational armature speed will cause a small current to begin to flow through the motor drive FETs 50. Once the flow of current is detected, it can be regulated through controlled pulse width modulation (PWM) at the motor drive FETs 50.
One primary disadvantage of the Zapi regenerative braking circuit is that high motor speed is required to initiate the regenerative braking current. The Zapi circuit requires a four-step process to begin regenerative braking:
1. Disengage forward contactor 42 and engage reverse contactor 44. PA0 2. Initiate plug braking by turning on the motor drive FETs 50 to "set" the flux direction in the field winding 40. PA0 3. Drop out the main contactor 10 to connect the negative terminal of the battery 20 to the top of the armature 30. PA0 4. Monitor the circuit for initiation of a regenerative braking current and signal the motor drive FETs 50 to control the regenerative breaking current.
In short, the Zapi regenerative braking circuit depends upon a weak magnetic field in the iron of the motor case to excite the armature current for regenerative braking. This eliminates the possibility of using non-ferrous metals in the motor case. Even more importantly, the armature current can only be generated in the Zapi circuit if the speed of the armature is great enough to overcome the voltage drops of the active and passive components in the circuit. This may require hundreds, or thousands of motor revolutions per minute, depending upon the motor and gearing ratios utilized, to begin regenerative braking.
There is, therefore, a need in the art for a more efficient regenerative braking apparatus and method which addresses the shortcomings of the available art. However, the applicant knows of no prior art which satisfies all of the aforementioned problems. More specifically, the following is the most relevant prior art known to the applicant.
U.S. Pat. No. 4,479,080 to Lambert is directed to an electrical braking control for DC series traction motors that initiates braking in a plug mode, transitions to regeneration mode, and returns to plug mode when regeneration braking is no longer efficient, with all switching carried out smoothly and efficiently without unduly wasting regenerative power. A series wound traction motor includes an armature winding, a field winding, a mechanical arrangement, and a battery source. A power regulating circuit is used and the field winding is arranged to be connected in either a forward or reverse direction by means of a plurality of contacts. A current shunt is connected in series with the motor and power source to supply a signal to the motor controller. Included in the circuit is a plugging diode, a free-wheeling diode, and a regenerative braking diode. Upon initiation of braking by the operator, the direction and brake logic circuit will switch from propulsion to plug braking mode. Signals from the shunt and the percent on-time controller are monitored by the regeneration control circuit and, if efficient, will switch into regenerative braking mode. When the motor speed decreases to the point that the regulator is operating at 100% duty cycle and the desired motor torque cannot be maintained, the braking operation will switch back to plug mode. While the Lambert device is somewhat effective, it has several disadvantages. One primary disadvantage is that, in the Lambert device the contactor 22 is switched from a closed position to an open position to force the transition from plug mode to regenerative braking mode. Using the contactor 22 as a switch between these modes exposes the contactor 22 to a welding risk since the contactor 22 cannot instantaneously go from the closed position to opened position. Another disadvantage in the Lambert device is its transition from regenerative braking mode to plug mode when a required motor torque cannot be maintained due to low motor rotation speed. As can readily be appreciated, this transition drains of the charge the battery stored during regenerative breaking. A further disadvantage in the Lambert device is its inability to regulate regenerative braking at high speeds.
U.S. Pat. No. 5,332,954 to Lankin is directed to a solid state electronic control for DC traction motors having a series or separate field. The control provides for regenerative braking at low motor speeds. The optimum configuration of a DC motor controller includes a plurality of MOSFET devices to connect the series wound or separately excited traction motor to the power source for propulsion or braking. During vehicle operation, a control logic circuit continuously pulls a brake sensor to determine if conditions are suitable for regenerative braking. When selected, the field circuit with the bridge arrangement of MOSFET devices can provide strong regenerative braking by supplying full field as required.
U.S. Pat. No. 4,730,151 to Florey, et al., is directed to a continuous field control of series wound motors, particularly for use in regulating electrical braking current in a direct current electric traction motor. A DC series motor with an armature and field winding is supplied power either in series or in parallel for operation in the running or braking mode. In normal, forward or reverse operation, the motor is connected in series mode with the battery supplying power to the armature connected in series with the field. Control is provided by a microprocessor controlling a chopper control in series with the motor. Field control mode, with the armature across the battery and the field in series with the chopper control, is used for maximum power in the running mode or for full control in regenerative braking mode.
U.S. Pat. No. 4,422,021 to Schwarz is directed to highly efficient recuperation of energy stored in a dynamo electric machine system using a control system that includes a controlled switch to initiate field current to start generation. The motor is controlled by a circuit including field reversing switches and a control switch. Upon braking, the field is reversed using the field switches and the controlled switch. A thyristor or transistor, is turned on, along with turn-off thyristor, to supply energy stored in capacitor to cause a field current in a winding. After the release of an impulse from the capacitor, the field current is maintained by a field diode until the generator action of the dynamo starts. A control unit will control the duty cycle of the cycling period, maintaining the current flow through the armature at an appropriate value.
Finally, U.S. Pat. No. 4,124,812 to Naito, et al., is directed to a braking control for a battery operated forklift that executes a regeneration mode, then plugging. Voltage from a battery is controlled by a chopping circuit and applied to the series circuit comprised of a motor field coil and motor armature. For braking, a regeneration contractor is switched and the armature and field are connected in parallel with one another, and in series with the chopping control across the battery for regenerative braking. After the velocity detector determines that the speed is down to a predetermined value, the braking is changed over to plugging.